In many in vivo clinical and research uses of light, incident radiation is applied to the interface between a turbid tissue and a transparent medium. Light that diffuses back to this interface and into the external transparent medium provides a noninvasive means to examine the parameters of photon diffusion within the tissue. Laser Doppler blood flow monitors, in particular, work on this principle. We have developed an analytical theory that enables calibration of such instruments from measurements of reflected light on the tissue surface. The theory relates such quantities as surface emission profile, sampling depth, and expected path length of migrating photons, to the scattering and absorption parameters of a tissue. The theoretically predicted wavelength dependence of those quantities has been verified experimentally. Also, dynamic light scattering studies have been performed in collaboration with Dr. N. Gershfeld (NIADDK/LBP). The major emphasis of this investigation has been on the structural transitions that occur in lipid-water suspensions. The movement of polystyrene beads has been used to probe the structure of the suspensions. Transformations have been seen in bulk dimyristoylphosphatidyl-glycerol (DMPG) water systems at the same temperatures where discontinuities in surface pressure are discerned in film balance studies. Below the transition temperatures the dispersed lipid seems to form a jelly-like slurry, whereas above those temperatures the suspensions contain lipid vesicles. Dr. Gershfeld has performed surface film studies that indicate similar transformations may occur in the growth of biological cells. Other dynamic light scattering studies have been initiated. A particularly promising collaboration (with G. Ehrenstein and J. Russell) concerns the release of peptides from neurosecretory vesicles.